This invention relates to apparatus for heating aluminum billets and the like to a temperature suitable for hot extrusion in a press.
In typical aluminum or other metal hot extrusion processes, metal billets of cylindrical shape are heated to a temperature suitable for extrusion. The preheated, cylindrical billet is then shaped into the desired form by forcing it through or squeezing it through a die under pressure. With aluminum processes, the billet is typically heated to a temperature of approximately 850.degree. F. prior to extrusion.
Various proposals have been made for furnaces or heating apparatus capable of preheating billets. For example, induction furnaces and direct flame impingement gas or oil furnaces have been employed. While serving to heat the billets to the desired temperature, these types of furnaces have been relatively inefficient from both a thermal energy standpoint and from a production standpoint. With gas and oil fired, direct or indirect impingement furnaces, heat is lost in the exhaust. Further, thermal inefficiency is reduced by the need for combustion air makeup. Also, with prior billet heaters or furnaces, additional energy losses have been encountered by heat conduction out of the furnace as a result of substantial use of through metal in the furnace designs. For example, prior billet heaters have employed a chain type conveyor and the like for passing the billets through the tunnel of the furnace with their longitudinal axes parallel to the direction of billet movement. These types of conveyors which extend through the exit opening of the furnace, provide substantial quantities of through metal for conduction of heat. Further, these types of conveyors prevent closing off of the exit opening with an insulated door structure. Such prior billet heaters or furnaces have also suffered from the need for large floor space requirements. This is due primarily to the fact that the billets are transferred through the furnace with their longitudinal axes parallel to the long axis of the furnace.
An example of one prior billet heater which solves some of the aforementioned problems may be found in commonly owned U.S. Pat. No. 3,994,678 entitled HEATER FOR BILLETS and issued on Nov. 30, 1976 to the present inventor. The heater disclosed in this patent includes an insulated tunnel structure and billet conveyor means transferring the billets through the various zones of the heater in tandem arrangement with their long axes extending parallel to the direction of travel. Each zone of the heater includes inner and outer chambers with the outer chamber supplied with high temperature gas at a pressure in excess of that of the inner chamber. The chambers are separated by baffles defining narrow slot-like aspirating throats through which secondary gases from a secondary source of air are jetted after entraining high temperature gases from the primary or outer chamber. The apparatus disclosed in this patent may be operated as a closed, recirculating type furnace using an electric heat source. The apparatus therefore represents a substantial improvement over prior devices through increased thermal efficiency.